The present invention relates generally to a method of hardening corrugating rolls and more particularly to a method of heat treating the flutes of corrugating rolls using a laser.
Corrugating rolls are typically used in machinery for manufacturing corrugated paperboard. The corrugating rolls are typically formed from an alloy steel, such as AISI 4340 steel, with a diameter of 12-16 inches and a length of 87 to 110 inches, and include longitudinally extending flutes which mesh with like flutes on a like roll to form the corrugations in paper webbing passed between them. The number and size of the flutes may vary, however, corrugating rolls are typically provided with between 33 to 39 flutes per linear foot having heights ranging from 0.044 to 0.187 inch. The flutes are sinuous in profile where the peeks or crests of the sinuous profile are referred to as "flute tips", the valleys are referred to as the "roots" and the inclines or oblique surfaces between the roots and tips are referred to as the "flanks".
Due to the meshing of the paper between the two corrugating rolls, the flute tips are subject to wear as a result of sliding contact with the paper, and the flute tips must eventually be machined back to their original dimensions or the corrugating roll must be replaced with a new one. In order to diminish this problem, the corrugating rolls are provided with a hardened outer surface. Typical methods of hardening the surface of corrugating rolls include plating the outer portion of the roll with a hard outer covering such as chrome or heat treating the entire roll, or heat treating only the tips of the flutes where the major portion of the wear takes place, as is described in U.S. Pat. No. 4,154,565, issued to Hyde et al and assigned to the assignee of the present application.
U.S. Pat. No. 4,154,565 discloses heat treating the flute tips of a corrugating roll by means of either a laser or an electron beam. In the process described by the patent for heat treating the roll by using a laser, an average hardening depth of 0.031 inch was obtained. The depth to which a flute can be hardened in the process described by this patent is limited by the temperature to which the flute can be heated by the laser beam without melting the surface material of the flute which is related to the distribution of the power density through the cross section of the beam.
A typical laser emits a beam having a circular cross section with a power density which varies according to a Gaussian distribution (or a higher order power density variation) such that the power density of the beam is greatest at the center of the beam and tapers off toward the radial edges. Thus, when a Gaussian laser beam is focussed upon the flute of a roll, the extent to which the flute may be heat treated by the beam is limited by the temperature which is reached by the portion of the flute impinged upon by the center of the beam, since this portion of the flute receives the greatest amount of energy in a given time and will be subject to reaching a melting temperature before the surrounding portions of the flute.
Therefore, there is a need for a method of heat treating the flutes of a corrugating roll to the maximum possible depth using a laser beam without causing surface melting or distortion of the flutes. More recently, it has been found advantageous to heat treat the flanks and roots of the corrugating roll also, as these surfaces, too are subject to wear. However, the flanks and roots can only be heat treated to the extent allowable for the heat treating of the flute tips. Said differently, the flute tips will be overheated and will melt prior to the complete heat treating of the flanks and roots.
It has been found that the flanks and roots require greater energy, than that of the flute tips. The flanks are not quickly heated due to their oblique nature, that is, the angle of incidence of the light source reduces the heat absorption for heat treating purposes. With respect to the roots, the surrounding mass of the corrugating roll prevents the roots from quickly heating thereby limiting the heat treating effect.
It is an object of the invention to provide a method and apparatus for heat treating corrugating rolls, where the entire flutes, including the tips, roots and flanks are heat treated.
It is a further object of the invention to provide such heat treating without overheating the flute tips, and without warping or otherwise distorting the roll.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.